The sensitivity advantage of Fourier-domain optical coherence tomography (OCT) over time-domain OCT is well established (see for example Choma et al. “Sensitivity advantage of swept source and Fourier domain optical coherence tomography,” Opt. Express 11, 2183-2189 (2003) and Leitgeb et al. “Performance of Fourier domain vs. time domain optical coherence tomography,” Opt. Express 11, 889-894 (2003)). Spectral-domain OCT (SD-OCT) and swept-source OCT (SS-OCT also referred to as time-encoded frequency domain OCT or optical frequency domain imaging) are the two most commonly used Fourier-domain OCT architectures
While there are many advantages of SS-OCT over SD-OCT, including less vulnerability to fringe wash-out and better roll-off sensitivity among others, SS-OCT systems with laser sources with high relative intensity noise (RIN) may not provide shot-noise limited performance like SD-OCT systems (see for example Yun et al. “High-speed optical frequency domain imaging,” Opt. Express, 11 2953-2963 (2003) and Yun et al. “High-speed spectral-domain optical coherence tomography at 1.3 μm wavelength,” Opt. Express 11, 3598-3604 (2003)). Typical experimental measurements of swept-source systems with lasers exhibiting RIN of ˜120 dB/Hz show sensitivity performance that is 10-12 dB less than theoretical shot-noise limited sensitivity. Incomplete RIN suppression might be one of the most significant factors for the reduced sensitivity performance in SS-OCT systems. A solution aimed towards increasing the sensitivity of SS-OCT systems could be one of the decisive factors for choosing SS-OCT system architecture for future OCT systems.
Chen et al. identified the non-uniform wavelength dependent splitting ratios of fiber optic couplers as the primary source of incomplete RIN suppression and proposed one means to reduce RIN suppression in an SS-OCT system (Chen et al. “Spectrally balanced detection for optical frequency domain imaging,” Opt. Express 15, 16390-16399 (2007)). The system employed by Chen et al. is shown in FIG. 1. As will be described below, the figure illustrates two different detection schemes, a conventional or standard hardware based balanced detection method (single channel data output) and their improved spectrally balanced detection scheme (dual-channel data output) that results in improved RIN noise suppression.
The box 101 in the top left corner of FIG. 1 shows the swept-laser source, in this case realized by using a rotating polygon minor based optical filter and semiconductor optical amplifier SOA. The light or radiation is divided into reference and sample arms at a 70/30 fiber coupler. The sample arm is directed to a slit lamp with an integrated X/Y scanner that scans the light over a sample of interest. The returned light from the sample is recombined with the source reference light at a 50/50 fiber optic coupler 102 and the resulting interference signal is divided into two output ports which can be used for either single-channel acquisition hardware balanced detection or dual-channel acquisition spectral balanced detection. Balanced detection is typically used in TD-OCT and SS-OCT to reduce RIN noise and fixed pattern noise artifacts (FPN). Fixed pattern noise may arise due to ripple modulations in the source spectral sweep. In a conventional SS-OCT balanced detection scheme (shown in dotted lines in the bottom left box 103 and labeled “1. Hardware balanced detection”), the two outputs from the 50/50 coupler are directed to the two input ports of a balanced detector.
A typical balanced detector consists of two reverse-biased photodiodes realized by applying voltages of same amplitude but opposite polarity as shown in FIG. 2 (see for example Houser et al. “Balanced detection technique to measure small changes in transmission,” Applied Optics, 33 1059-1062 (1994)). The light incident on each of the photodiodes PD1 and PD2 generates photocurrents which are subtracted from each other at the junction 201 of the two photodiodes. The subtracted current signal is then converted to a voltage by a transimpedance amplifier U1. In the case of SS-OCT and TD-OCT, balanced detection results in addition of the interference signals while the DC component (due to source reference light), and hence the RIN noise, is subtracted and nullified.
To achieve optimized balanced detection with the least amount of RIN noise and FPN artifacts, a uniform splitting ratio over the whole spectral sweep of laser is necessary. However, as box 104 inset in the upper right hand corner of FIG. 1 illustrates for the signal measured at each detector of the balanced detector individually A and B, the splitting ratios of fiber optic couplers vary with wavelength. Chen et al. found that the wavelength dependent splitting ratio deviated by ±12% over a wavelength range of 60 nm. Chen's proposed detection scheme, shown in solid lines in the bottom left hand box 103 of FIG. 1 (and labeled “2. Spectrally balanced detection”), involves directing the divided reference signal to two detectors and digitizing the electrical signals separately. The balanced detection in this scheme is performed in the digital domain with an optimized balancing algorithm. The ratio of the two channels was measured and a compensation function was calculated to describe the channel ratio according to: R(λ)=(Sch1(λ)/Sch2(λ)). The balanced detection was then performed by rescaling the output from two detectors and subtracting one from another according to:Sbal=Sch1(λ)/√{square root over (R(λ))}−Sch2(λ)·√{square root over (R(λ)·)}  (1)
Moon et al. suggested an approach similar to Chen et al. where they used multi-channel acquisition (up to three channels) and corrected the signal in digital domain after applying various normalization schemes. (Moon et al. “Normalization detection scheme for high-speed optical frequency-domain imaging and reflectometry,” Opt. Express 15, 15129-15146 (2007)).
While reducing RIN noise, these methods have several drawbacks including the requirement for twice the data bandwidth (dual channel detection), additional computation load for post processing, higher detector noise coming from the addition of multiple detectors and the requirement of higher bit-depth for each channel due to DC offset. Although introducing the need for dual channel detection, these methods only sample a single interferometer path.
Electronic methods of balancing an optically unbalanced signal prior to digitization have been shown (For example See U.S. Pat. No. 5,134,276 and corresponding product Nirvana™ (Newport Corporation)). This technique can be thought of as an analog calculation of a correction parameter applied to one of the photodiode currents prior to subtraction at the node and digitization. US Publication No. 2011/0228280 also describes methods of electronically balancing a single interferometer.
In light of the limitations of the prior art, here we describe both optical hardware and electronic based solutions to spectrally filter and attenuate the source reference light in optical coherence tomography in an effort to reduce RIN and FPN noise in OCT systems. For optical hardware based solutions, the means for spectral balancing can be located in the interferometer itself, or in one of the paths directing light to the balanced detection input ports, or in both locations to optimize matching of spectral and power characteristics of the source reference light prior to balanced detection (i.e. prior to electric signal current generation at the photodiodes and data acquisition). In addition to the optical hardware means, we present some new electronic balanced detection schemes in which the current generated by at least one of the photodiodes is divided or amplified dynamically to nullify the signal current due to unbalanced source reference light at the subtraction node of the modified balanced detector. Such designs can compensate for the non-uniform wavelength dependent splitting ratio of fiber optic couplers in SS-OCT systems. The majority of noise in an OCT system arises from the reference light as it is typically orders of magnitude larger than the sample light. Improved spectral matching of the source reference light in the two input arms of the balanced detector results in efficient FPN suppression and can increase the sensitivity of SS-OCT systems closer to shot-noise limited performance. The systems and methods described herein have the advantage of using half the data bandwidth, allowing for single channel detection and higher dynamic range (hence less bit-depth), and requiring no post-processing steps for RIN suppression compared to the solutions provided in the past.